The invention relates to a method of downhole drilling and apparatus therefor such as an electrically powered bottom hole assembly for use in coiled tubing drilling (CTD) applications.
Simple CTD services are known using hydraulic motors to provide the rotational torque in the drill bit using hydraulic pressure of a suitable fluid. Whereas initial efforts at CTD were based around remedial work in an existing wellbore, the technology is now used to drill wells from surface and to sidetrack existing wells. Both overbalanced and underbalanced drilling techniques have been evaluated along with advances in directional drilling technology.
However there are significant drawbacks with the existing hydraulic motor systems. They have a very low durability, due mainly to the failure of seals and generally to the problems of transmitting high pressure over long distance in a well. Such failure requires withdrawal of the whole string from the well. Also, conventional coiled tubing drilling techniques have a limited choice of drilling mediums.
It is therefore an object of the present invention to provide a method of downhole drilling and apparatus therefor which alleviates or overcomes at least some of these disadvantages.
According to the invention there is provided an apparatus for downhole drilling of wells comprising;
a drilling unit comprising a drill bit for penetrating into a rock formation,
a motor arranged to drive the drill bit, the motor including a hollow shaft which permits the passage of fluid therethrough,
tubing upon which the drilling unit and motor are suspended,
control means which monitor and control the action of the motor and/or drill bit, and
cable means disposed along the tubing.
Preferably, the tubing is coiled tubing. Preferably the cable means is disposed within the coiled tubing. Preferably the hollow motor is a brushless DC motor providing direct control over the speed and torque of the drill bit. Preferably at least one sensor is provided between the motor and the drill bit. Preferably the sensor or sensors include a rock type sensor such as an x-ray lithography sensor.
The control means provides the required control over the motor in terms of its speed and torque to prevent stalling of the motor and to provide the most desirable rate of progress of the drilling process. The control means may be provided with direction output means to control the direction of the drilling by input to a directional drilling control means. Similarly, the control means may be provided with thrust output means to control the level of thrust of the drilling by input to a thruster control means. Preferably the thrust means include a plurality of eccentric hub type thrusters.
Also according the present invention there is provided a method of downhole drilling using an apparatus as defined above.
Mud may be pumped down the inside of the coiled tubing, through the hollow shaft of the motor, and to the bit in order to wash the cuttings away from the bit and back up the well through the annulus formed between the side of the well on the one hand and the outside of the coiled tubing and the motor on the other. Or alternatively, mud may be pumped down the annulus formed between the side of the well on the one hand and the outside of the coiled tubing and the motor on the other, and thence to the bit in order to wash the cuttings away from the bit and back up the well through the hollow shaft of the motor and the inside of the coiled tubing.